1. Field of the Invention
The present invention relates to an optical device, an optical scanner, and an image formation apparatus.
2. Description of the Related Art
For the purpose of size reduction, optical devices employing a torsional vibrator have been used in laser printers for conducting drawing with optical scanning. For example, Japanese laid-open patent publication No. 7-92409 discloses such an optical device.
An optical device disclosed by Japanese laid-open patent publication No. 7-92409 has a movable plate made of silicon, a light reflection portion provided directly on the movable plate, and a pair of torsion springs for supporting both sides of the movable plate. The light reflection portion is made of aluminum. When the pair of the torsion springs are torsionally deformed, the movable plate is rotated (vibrated) so as to conduct optical scanning. At that time, most of light applied to the light reflection portion is reflected from the light reflection portion.
However, a light reflectivity at the light reflection portion cannot perfectly be made 100%. Accordingly, in such an optical device, a portion of light applied to the light reflection portion is converted into heat, which increases the temperature of the movable plate.
Thus, when such an optical device is used for a long period of time, deformation such as a warp may be caused to the movable plate due to heat depending upon the shape of the movable plate or the material of the light reflection portion. In such a case, the flatness of the light reflection portion is deteriorated. Further, properties of a material of the torsion springs may be changed by heat transferred from the movable plate, so that a spring constant of the torsion springs is changed. The deterioration of the flatness of the light reflection portion and the change of the spring constant of the torsion springs make it difficult to achieve a stable drive (drawing).